Rechargeable electric storage batteries of many different kinds are known, such as nickel-cadmium, nickel metal hydride, nickel-iron, lithium, silver-cadmium and deep-cycle lead acid batteries. Deep-cycle lead acid batteries differ from SLI (starting, lighting, ignition) lead acid batteries used, e.g., in conventional automobiles; SLI batteries are not designed or constructed to withstand repeated cycles of substantial discharge and recharge, and so are not rechargeable batteries in the sense of this invention.
It is known, such as from U.S. Pat. Nos. 4,392,101 and 4,503,378, that there are certain characteristics of a rechargeable battery, regardless of kind, which change during recharging of the battery in ways which signal either that the battery is fully charged or that it is at a relatively predictable point short of but near a state of full charge. Those patents, as well as other publications, describe equipment and techniques for monitoring those characteristics and for detecting certain events, conditions or states of them, and using such detections either to terminate the battery charging process or to continue charging for preset times or in preset ways. Those preset ways typically use charging processes different from those in use at the time of the detected event. Those charging event detection techniques are known as inflection analysis methods because they rely on the detection of certain inflection points in time-based curves which describe the change in battery voltage or battery current, e.g., during the charging process. While inflection analysis as described to date works well to control recharging of most kinds of rechargeable batteries, inflection analysis as heretofore described has been found not to serve satisfactorily for controlling recharging of flooded deep-cycle lead acid batteries in which the battery electrolyte is a liquid (typically sulfuric acid) unconfined in any supporting matrix such as a gel.
Flooded deep-cycle lead acid batteries are widely used as energy sources for electrically powered vehicles such as golf cars, fork lift trucks, and scissor lift vehicles. They also are used in uninterruptible power supplies in hospitals and other buildings and facilities, and as components of photovoltaic power installations. The reasons why inflection analysis techniques as heretofore described are not satisfactory for controlling recharging of flooded deep-cycle lead acid batteries can be understood from the use of such batteries in electric golf cars, as an example.
Electric golf cars are powered by sets of 4, 6 or so flooded deep-cycle lead acid electric batteries. At a given golf course, there is a fleet of such golf cars available for use by golfers. Different cars in the fleet may have older batteries in them than other cars in the fleet. Certain cars may be used more frequently than others. Some cars may be used longer on a given day than others. Some cars may be subjected to more strenuous usage conditions on a given day than others, depending on the circumstances of the using golfers or differences in traversed terrain, among other reasons. Also, it is well known that even if all batteries in the fleet are from the same manufacturer and are of the same nominal age, there still will be meaningful variations between batteries of kinds which can affect battery performance, life and, importantly, how they respond to recharging processes. As a consequence, at the end of a day when the golf cars in that fleet are to be recharged, there can be significant differences between the discharge states of the batteries from car to car, and consequent meaningful differences from car to car in how the batteries need to be charged. Fleet-wide uniform recharging procedures either will cause some batteries to be insufficiently recharged or, more likely, substantial numbers of the batteries will be materially overcharged. Material overcharge of such a battery reduces battery life. Very commonly, the persons employed to recharge fleets of golf cars have no understanding of the effects of substantial overcharge and how to determine when it is occurring. Therefore, it is desirable that the batteries used in electric golf cars be recharged by equipment and processes which avoid substantial overcharge and do so in ways which inherently accommodate and deal with differences between batteries due to discharge state, age, and manufacturing variations, among other factors.
Deep-cycle lead acid batteries are designed to withstand repeated cycles of substantial discharge from a fully charged state and of recharge from a discharged state to a state of full charge. As compared to other kinds of rechargeable batteries which do not use liquid electrolytes, the liquid acid electrolyte of flooded deep-cycle lead acid batteries presents special conditions which require that a given battery, or a given set of a small number of batteries repeatedly used in combination with each other, be recharged in a way which provides a controlled overcharge related in extent to the state of the battery at the time a recharge event is commenced. Stated differently, effective recharge of a flooded deep-cycle lead acid battery ideally should include a controlled overcharge determined by the amount of energy removed from (discharged by) the battery during its last preceding duty cycle (period of use following the last prior charging event). The reason is related to what happens to the liquid electrolyte during the prior duty cycle and the following recharge event.
As a cell of a lead acid battery discharges, the acid ions in the electrolyte move to the cell electrodes and oxygen atoms move from the active material of the cell into the electrolyte to form water with the electrolyte hydrogen ions. As a consequence, the electrolyte acid becomes progressively more diluted and its specific gravity progressively approaches 1.0 from a higher starting specific gravity. As the cell is recharged, that ion exchange process is reversed to produce regeneration of the electrolyte acid and the active material. If the electrolyte is present in the cell as a free liquid (i.e., the cell is flooded), as opposed to being present in a gel matrix, the regenerated acid, being heavier than the dilute electrolyte, sinks to the bottom of the cell as it is created. As the recharging process continues, more and more concentrated regenerated acid collects in the bottom of the cell. At the point at which the cell active material has been fully regenerated, the cell is theoretically fully recharged on a Coulombic basis. However, the cell is not in good condition for use to deliver stored electrical energy because of the stratification of the electrolyte. The electrolyte is not of uniform acidity throughout the cell and so the regenerated acid electrolyte is not in uniformly effective contact with the regenerated active material over the full area of the regenerated active material; if the cell were to be called upon to discharge at that point, the discharging electrochemical process will occur predominantly in the lower part of the cell where the electrolyte acid is overly concentrated. The cell will not discharge energy at the levels desired, and the over concentrated acid in the bottom of the cell will cause overly rapid degradation of the adjacent active material. The consequence is under performance of the cell in a manner which materially reduces cell life.
In the portion of the recharge process for a lead acid battery cell which immediately precedes full regenerative restoration of the active material, gas is generated in the cell as a normal part of the recharge process. The gas bubbles rise through the electrolyte to the top of the cell and, in the process, induce circulation (stirring) of the electrolyte in the cell. However, if the recharge process is terminated at the point of full regeneration of the active material, the amount of gas generation which will have occurred will be insufficient to stir the electrolyte adequately to cause it to be of uniform acid concentration (uniform specific gravity) throughout the cell. For that reason, it is common practice to continue the process of recharging a flooded deep-cycle lead acid battery beyond the point of full recharge, i.e., to extend the gas generation process for a time to achieve adequate stirring of the regenerated electrolyte. That is, the cell is intentionally overcharged.
Current practice is to overcharge such batteries, which include a number of cells, by a predetermined amount which is defined to be adequate to fully stir the electrolyte in the cell or cells which need the most stirring; that definition of the predetermined amount of overcharge is based on the assumption that the cell has been maximally discharged in its previous duty cycle and that the cell has certain properties of age, condition and temperature. However, as shown above in the discussion of the operation of a fleet of electric golf cars, that assumption is not apt for a substantial portion of batteries requiring recharge. As a result, reliance upon that assumption about the amount of overcharge to be applied in the terminal stages of recharging flooded deep-cycle lead acid storage batteries causes a substantial number, if not the majority, of such batteries to be meaningfully overcharged. Meaningful overcharge of such a battery, especially if repeated more than a few times, substantially reduces the effective life of such a battery.
The foregoing description provides a foundation for understanding how existing descriptions of inflection analysis techniques for controlling battery recharge processes are deficient when applied to the recharging of flooded deep-cycle lead acid storage batteries.
U.S. Pat. No. 4,392,101 is an early description of the use of inflection analysis in controlling recharging of rechargeable batteries. It teaches that rechargeable batteries in general have broadly similar response characteristics to recharging processes. It teaches that if battery voltage or current, e.g., is plotted graphically against time during recharge, the resulting voltage/time or current/time curves will have broad similarities. After initiation of the charge process, irrespective of the particular materials used to define a battery cell, those curves will manifest at least a pair of inflection points in which the graph line reverses curvature, i.e., is inflected. It is disclosed that those inflection points signal or denote different phases of the battery's response to applied charging energy and, for each type of cell, those inflections occur at relatively predictable times in the process, either before or at the time of the battery reaching a state of full charge. It is disclosed that the predictability of the inflection point occurrences is generally unaffected by (happens without regard to) factors such as the actual voltage of the battery, individual cell characteristics, individual charging history, or actual ambient temperature conditions. That patent discloses that the inflection points can be identified by observing the state or character of the first or second derivative with respect to time of the battery characteristic (voltage or current) being monitored. More particularly, it teaches that a graph of the second derivative will cross the zero axis (the sign of the derivative will change from positive to negative, or vice versa) at least twice during the charging process, and the second zero axis crossing of that derivative either will occur at the time the battery reaches fill charge or will occur at some interval shortly before fill charge is achieved. However, in the instance of lead acid batteries, that patent does not attempt to describe when the second time-based derivative of voltage occurs relative to full charge. The principal descriptions of that patent are in the context of nickel-cadmium batteries where recharging is terminated a preset time after that second zero-axis crossing of that derivative has been detected. Nickel-cadmium batteries do not use a variable density electrolyte which is present as a part of the chemical process and so such batteries do not benefit from or require any measure of overcharge.
U.S. Pat. No. 4,503,378 applies inflection analysis recharging controls to nickel-zinc batteries and discloses that, for that type of battery, recharging is to be terminated upon the occurrence of the second instance of sign change (zero axis crossing) of the second derivative of battery voltage with respect to time. It also observes that, at the same time as the second derivative crosses the zero axis from positive to negative, the value of the first derivative of battery voltage with respect to time is at a maximum or peak value, a fact which enables the second derivative's zero crossing to be confirmed.
The article titled “Charge batteries safely in 15 minutes by detecting voltage inflection points” appeared in the Sep. 1, 1994, issue of EDN Magazine. That article focuses principally upon fast recharging of nickel-cadmium batteries. It comments that inflection analysis also applies to lead acid batteries. In that connection, it states “In lead-acid batteries, the second dV/dt inflection occurs at a predictable interval before the batteries reach full charge, but from the battery's Ahr capacity rating, you can easily derive the duration of the incremental charging needed to achieve full charge.” That statement does not contribute, for at least two reasons, to a solution to the problem of how to efficiently, reliably and effectively charge a flooded deep-cycle lead acid battery, without meaningfully overcharging it, in terms of the battery's true need for recharge. First, a lead acid battery's Ahr (ampere-hour) capacity rating is not a precise value which can be determined accurately from engineering information. Rather, it is a value which a battery manufacturer assigns to a model or type of battery as a result of business factors peculiar to the manufacturer, such as marketing objectives, warranty policies, and other factors. A battery's ampere-hour capacity rating is merely a manufacturer's statement of the expectable performance, perhaps under unspecified conditions, of an average battery of that kind or type. It has no reliable relation to the charging needs of a particular battery after completion of a particular duty cycle, i.e., its depth of discharge before experiencing a recharging event. Second, the ampere-hour capacity rating is a value which needs to be known from a source other than the battery itself. What is needed is a way to charge a flooded deep-cycle lead acid battery using information, derived from the battery itself, which describes the battery's discharge state and which is usable to overcharge the battery only enough to stir the regenerated electrolyte adequately.
Neither of the patents cited above nor the EDN Magazine article consider the state of battery discharge before a recharging process is commenced. They impart no knowledge about how information about that discharge state can be used to control recharge of that battery. However, apart from those descriptions it is known (such as from U.S. Pat. No. 6,087,805) to physically attach to a battery, such as a battery in a golf car, an integrating ampere meter (ampere hour meter) which travels with the battery at all times. When the battery is connected to a charger following the battery duty cycle, the “on board” ampere hour meter is connected to the charger so it can communicate to the charger the value of ampere hours removed from the battery during that last duty cycle. That information is applied in the charger to a computing and control device which computes the total charge to be delivered to the battery by multiplying the metered value of ampere hours by the desired factor (for example 1.10 or 110%)that has been found to produce sufficient stirring in the electrolyte. A computing and control device in the charger then monitors the ampere hours returned to the battery by the charger. When the calculated value for the charge return is reached, that computing and control device instructs the charger to terminate the charging process. While this approach is effective, it suffers from the added complexity of communicating data to the charger from the ampere hour meter which is associated with the battery. That approach also suffers from the added expense of equipping every battery, or every operational set of batteries, with its own captive ampere hour meter which must be specially constructed to survive in the environment of the battery. That approach is independent of inflection analysis and has apparent practical problems in the field.
It is apparent, therefore, that a need exists for the availability of equipment and procedures which can be used effectively, efficiently and reliably by persons having little or no knowledge of battery technology to adequately recharge flooded deep-cycle lead acid batteries without meaningfully overcharging any one or small group of batteries. Such equipment and procedures, to satisfy that need, should effectively address and conform to the actual recharge and electrolyte stirring needs of a battery or of a defined small group of batteries. The term “defined small group” means a number of batteries, such as those installed in a given electric golf car, which most probably will be of the same age, will have experienced the same usage history, and will have shared the same duty cycle in the interval between last being recharged as a group and the recharge event of interest.